1. Field of the Invention
The invention relates to a secondary battery. More particularly, the invention relates to a secondary battery that is provided with a wound electrode assembly in which a positive electrode sheet, a negative electrode sheet, and a separator are stacked and wound. The positive electrode sheet is provided with a positive electrode active material layer on a long positive electrode current collector. The negative electrode sheet is provided with a negative electrode active material layer on a long negative electrode current collector. The separator is interposed between the positive electrode sheet and the negative electrode sheet.
2. Description of Related Art
Secondary batteries such as lithium ion secondary batteries and nickel hydride batteries have in recent years been preferentially used as vehicular power sources and power sources for personal computers and portable and mobile electronic devices. Lithium ion secondary batteries in particular are light weight and have a high energy density. As a consequence, they are growing in importance as, for example, high-output power sources for installation in vehicles and as power sources for power storage systems. One type of conventional battery has a battery structure that is provided with a wound electrode assembly made by stacking a long positive electrode sheet and negative electrode sheet with a separator interposed therebetween and winding this into a spiral configuration. The reaction area between the positive and negative electrodes can be increased by using a spiral configuration for the electrode assembly. This makes it possible to raise the energy density and obtain high outputs.
The positive electrode sheet in this wound electrode assembly is generally fabricated by forming positive electrode active material layers by supplying a positive electrode active material layer-forming composition containing a positive electrode active material to both sides of a positive electrode current collector. This positive electrode active material layer-forming composition can be prepared, for example, as a paste or slurry. The negative electrode sheet is fabricated by forming negative electrode active material layers by supplying a negative electrode active material layer-forming composition containing a negative electrode active material to both sides of a negative electrode current collector.
Here, in the case of high-output service such as installation in a vehicle, an uncoated region where the active material layer is not formed (the active material layer-free region) is provided on these electrode sheets (i.e., the positive electrode sheet and/or the negative electrode sheet) and is used as a current collection region. In order to raise the current collection efficiency, this uncoated region is disposed as a strip on both sides at one edge in the width direction that is orthogonal to the length direction of the electrode sheet. In addition, the uncoated regions of the two electrode sheets are disposed in a staggered manner whereby the uncoated region of the positive electrode sheet and the uncoated region of the negative electrode sheet project out toward opposite sides in the width direction, and the wound electrode assembly is then fabricated by winding using these electrode sheets. When a secondary battery provided with a wound electrode assembly having such a configuration is subjected to repetitive charge/discharge, the expansion and contraction of the active material accompanying charge/discharge can cause deformation, e.g., buckling and so forth, in the wound electrode assembly, and the properties may then deteriorate. In view of these circumstances, the invention provides a secondary battery that exhibits a high ability to withstand charge/discharge cycling.